Review of the Biology and Population Dynamics of the Blue Crab, Callinectes sapidus, in Relation to Salinity and Freshwater Inflow Gandy, R. L., Crowley, C. E., Machniak, A. M. and Crawford, C. R. Florida Fish and Wildlife Conservation Commission Fish and Wildlife Research Institute Crustacean Research Department 100 Eighth Avenue Southeast St. Petersburg, Florida 33701-5095 Prepared for The Southwest Florida Water Management District 2379 Broad Street Brooksville, Florida 34609-6899 PO# 10POSOW1595 February 2011 Task I Reproductive Biology and Population Dynamics of the Blue Crab Callinectes sapidus The blue crab, Callinectes sapidus, is an ecologically and economically important component of tropical, subtropical, and temperate ecosystems. The range of the blue crab is broad, extending from Argentina in the southern hemisphere to Massachusetts in the northern hemisphere. Blue crab occurrences in Nova Scotia, Denmark, Northern Adriatic, the Black and Mediterranean seas, and central Japan (Norse 1977, Williams 1974) have also been reported. Blue crabs are fundamental components of estuarine food webs, functioning as both predator and prey. The blue crab’s role as predator is arguably, one of the most important biotic determinants of community structure in estuarine ecosystems (Mansour 1992, Guillory 2001 (b)). Habitat selection of the blue crab is dependent on the particular physiological requirements of each life history state in its complex life cycle (Guillory et al. 2001 (a), Perry 1984). Blue crabs are present in planktonic, nektonic, and benthic stages, in habitats ranging from offshore to near-shore estuarine phases (Guillory et. al 2001 (a)). One of the most important habitats are the low salinity river and estuarine systems where mating occurs (Guillory et al. 2001 (a)). Throughout the months of March to November, in the Gulf of Mexico (Johnson 1999), immature females who have not reached their pubertal molt, seek out low salinity estuaries (<15 ppt) with high densities of mature males for mating (Guillory et al. 2001 (a)). While in the brackish waters of the upper estuary the juvenile females will molt and subsequently mate (Johnson 1999). Following successful mating, the interval between mating and egg extrusion varies between two and nine months. When mating occurs in the spring and summer a two month interval is common. However, blue crabs that mate during fall and winter spawn during the following spring or when water temperatures rise to about 19ºC (Steele 1982). Year round spawning of Florida blue crab has been recorded with specific peaks in spring and fall. Mature mated female blue crabs are catadromous, migrating from hyposaline waters <30 ppt to higher salinity water in the lower estuary and offshore to spawn (Hines et al. 1987, Steele and Bert 1994). During the ebb tide females will take advantage of the outgoing waters and make their way out of the estuary within the spring, summer, and fall months (Perry and Stuck 1981, Steele and Perry 1990, Johnson 1999). The movement of ovigerous females has been documented both out of estuaries and along the coast of Florida. In a 1989 tagging study, Steele, observed a single–gender specific migration North of Tampa Bay along the coast (Steele 1991). These findings are similar to studies of coastal areas south of the Apalachicola River conducted by Oesterling and Evink (1977). Originally, Oesterling (1976) hypothesized that females engaged in a mass migration to reach spawning grounds south of the Apalachicola River. This hypothesis presented a scenario where the low salinity flow from the Apalachicola River would transport larvae offshore to the Loop Current for redistribution of immature crabs to south Florida estuaries. Over turning this hypothesis was the documentation that newly hatched larvae require salinities in excess of 22 ppt (Costlow and Bookhout 1959, Sulkin and Epifanio 1975). Results of the 1989 Steel tagging study suggested the reduced salinity from the outflow of the Apalachicola River at Apalachee Bay acts as a freshwater barrier to the migrating females, preventing westward emigration rather than an offshore transport mechanism for larvae (Steele 1991). The occurrence of female migration is not unique to the western coast of Florida. In northeastern Florida, mature females in the St. John’s River were tagged and their location documented upon capture. Their gradual movement out to sea was evidenced by collection throughout the spring and early summer months in which 96% of females were obtained downstream of the release point (Tagatz 1968). In addition, the highest abundance of egg bearing females was captured in July through September at distances of 5-6 km offshore of the river mouth (Tagatz 1968). During the summer and fall months the arrival of a mature, mated and egg bearing females in the high salinity waters (>30 ppt) of the lower estuary stimulates the larvae of mature egg masses to hatch (Gunter 1950, Daugherty 1952, More 1969, Perry 1975, Tankersley 2002). Hatching occurs during ebbs tides, allowing larvae to be swept seaward (Epifanio et al. 1989, Tankersley 2002). Newly hatched larvae are transported from the lower estuary to offshore waters via surface currents (Johnson 1999). Laboratory test have determined that while offshore, the larvae will complete seven zoeal life stages over approximately 30-50 days (Costlow and Bookhout 1959). Subsequently, the final zoeal larval stage metamorphoses to the megalopae stage which lasts from 6-20 days (Costlow and Bookhout 1959). Movement of megalopae back into the estuary is dependent on a variety of physiographic parameters, salinity regimes, tidal periodicity, long-term water level cycles, wind regimes, and coastal currents (Rabalais 1995, Guillory 2000). These physical factors or forces have been shown to affect the harvest of adult blue crabs in subsequent years (Johnson 1999, Heck et al. 2001). Megalopae are able to attain up-estuary transport through tidally-related vertical migration, also known as Flood Tide Transport (Tankersley 2002, Rablais 1995, Olmi 1994, 1995). This migration allows for up-estuary movement using tidal flood currents for settlement in shallow nearshore areas that are crucial for food and refuge from predators (King 1971, More 1969, Perry 1975, Perry and Stuck 1982, Johnson 1999, Heck et al. 2001). Depending on the strength and timing of physical forces, the megalopae will be transported various distances into the estuary. This reinvasion of the estuaries occurs from March to November, with highest abundances in the late summer and early fall (Heck et al. 2001). The primary habitats where settlement and metamorphosis into first crab stage (approximately 2-3 mm in size) occurs are marshes and seagrass beds (Tagatz 1968). In fact, 90% of juveniles in any given area occur in seagrasses or marshes (Orth et al. 1990, Heck et al. 2001, Perry 1975), and up to 95% are less than 25 mm (Heck et al. 2001, Perry 1975). The highest abundances of juveniles occur in beds of submerged aquatic vegetation (SAV) which includes Zostera marina, Ruppia maritime and Halodule wrightii marshes. These appear to be the primary nursery habitats for the earliest juvenile instars. The occurrences of juveniles in sites with SAV are as much as five times higher than marsh sites containing Spartina alterniflora (Murphy et al. 2007, Heck et al. 2001, Moksnes and Heck 2006). In juvenile abundance studies performed by Heck et al. (2001) in the northern Gulf of Mexico the greatest abundance of early juveniles collected were in the lower bay sites with an average salinity of 23 ppt. Based on the increasing densities of larger juveniles in lower salinity waters, the data suggests movement up the bay toward lower- salinity waters into oligohaline marshes and SAV beds (Heck et al. 2001, Orth and van Montfrans 1987, Thomas et al. 1990, Williams et al. 1990). In general, juvenile distribution occurs over a broad salinity range, often times based on estuary location in the Gulf of Mexico. From Guillory et al. (2001 (a)): Although juvenile crabs occur over a broad salinity range, they are most abundant in low to intermediate salinities characteristic of middle and upper estuarine waters. Daud (1979) found early crab stages (5-10 mm) in shallow brackish/saline waters and observed movement into fresher waters in larger juveniles. Swingle (1971), Perret et al. (1971), Christmas and Langley (1973), and Perry and Stuck (1982) determined the distribution of blue crabs (primarily juveniles) by temperature and salinity using temperature-salinity matrices (Table 3.1) [Table 1 in this review]. Both Perret el al. (1971) and Swingle (1971) found maximum abundance in salinities below 5.0‰. In contrast, Christmas and Langley (1973) and Perry and Stuck (1982) found highest average catches associated with salinities above 14.9‰ in Mississippi. Based on one year of bag seine data, Hammerschmidt (1982) found no direct relationship between catches of juvenile crabs and salinity in Texas. Walther (1989) examined the relationship between recruitment of juvenile blue crabs (as measured by catch per unit of effort in 16 ft trawl samples) in Barataria Bay, Louisiana and salinity. He found a significant negative relationship between February-May blue crab catch per unit effort and salinity for the same time period (R2=0.80). Although salinity influences distribution, factors such as bottom type, food availability and competition also play a role in determining distributional patters of juvenile blue crabs. Table 1. Juvenile Blue Crab Temperature-Salinity Matrices. (Reproduced from Guillory et al. 2001 (a)). Habitat partitioning by juveniles is evident in many estuarine systems throughout the Gulf of Mexico. Seasonally, males and females will be distributed with respect to salinity and sex (Guillory et al. 2001 (a)). In Tampa Bay males are most abundant in the upper bay. Steele and Bert (1994) noted that the percentage of males is inversely related to the annual mean salinity. These findings are consistent with studies in the St. John’s River, Chesapeake Bay, and Louisiana coasts where male abundance is typically highest in the upper bay where lower- salinity regions occur (Tagatz 1968, Jaworski 1972, Hines et al.